In the time zoo-animals were still coming from the wild normaly animals which were living in relative easy acceseble areas were caught for the animal-trade. With Gorillas it was also the case that mainly young / babies were sold to zoos because adults were to difficult to catch. Countries like Gabon and Cameroon have / had good acces so gorillas living there ( = western lowland gorillas ) ended-up in the animal-trade.
Mountain and Eastern lowland gorillas live in much more difficult to reach areas and the young / baby-animals had to be brought to the coast (transport in those days was mainly by ship ) and because of the distance only very few animals made this journey without dying.
In the mid-1950-ties Congo was still a colony of Belgium and thats why most of the very few Eastern lowland gorillas which were brought to western zoos ended up at Antwerp Zoo.
In the mid-1980-ties an attemp to bring some Mountain gorillas to a zoo in Germany ( as can be seen in the movie "Gorillas in the Mist" ) ended with the death of the 2 animals - if I remember it right - and thats why further attemps didn't take place after that.